1. Field of the Invention
The present invention relates to an impedance converting circuit preferably for use in an antenna device and other devices. In particular, the present invention relates to an impedance converting circuit that provides matching in a wide frequency band, and to a communication terminal apparatus that includes the impedance converting circuit.
2. Description of the Related Art
In recent years, communication terminal apparatuses, such as cellular phones, have often been required to support communication systems, such as Global System for Mobile Communications (GSM) (registered trademark), Digital Communication System (DCS), Personal Communication Services (PCS), and Universal Mobile Telecommunications System (UMTS) as well as Global Positioning System (GPS), wireless LAN, and Bluetooth (registered trademark) and others. Therefore, antenna devices in such communication terminal apparatuses are required to cover a wide frequency band of 700 MHz to 2.7 GHz.
Antenna devices for a wide frequency band typically have a passive matching circuit at a feeding end of an antenna element, as disclosed in Japanese Unexamined Patent Application Publication No. 2004-336250 and Japanese Unexamined Patent Application Publication No. 2006-173697. The passive matching circuit includes passive elements and is formed by an LC parallel resonant circuit or an LC series resonant circuit. Antenna devices that have an active matching circuit including active elements, as disclosed in Japanese Unexamined Patent Application Publication No. 2000-124728 and Japanese Unexamined Patent Application Publication No. 2008-035065, are also known as antenna devices for a wide frequency band.
In general, the impedance of an antenna element varies depending on the size and shape of the antenna element, the surroundings and others. Therefore, the passive matching circuits disclosed in Japanese Unexamined Patent Application Publication No. 2004-336250 and Japanese Unexamined Patent Application Publication No. 2006-173697 need to be adjusted for each model of communication terminal apparatus. In particular, it is not easy to optimize the impedance for each of a plurality of frequency bands. Adjusting the impedance for each of a plurality of frequency bands tends to increase the number of elements included in the matching circuit. Such an increase in the number of elements in the matching circuit may lead to increased insertion loss and make it difficult to obtain a sufficient gain.
The active matching circuits disclosed in Japanese Unexamined Patent Application Publication No. 2000-124728 and Japanese Unexamined Patent Application Publication No. 2008-035065 also need to be adjusted for each model of communication terminal apparatus. Additionally, the active matching circuits require a circuit for controlling a variable capacitance element, that is, require a switching circuit for switching the frequency band. As a result, the circuit configuration tends to become complicated. Also, due to high loss and large distortion in the switching circuit, it may not be possible to obtain a sufficient gain.